Temperature jump relaxation techniques are being used to determine forward and reverse rates of complexation between spinach ferredoxin and ferredoxin: NADP reductase and to determine rates of electron transfer with the protein-protein complex. Parallel studies in the presence of NADP(H) are designed to determine the role of this cofactor as an allosteric effector by ascertaining its effects on rates and equilibria of complexation and/or electron transfer. Redox equilibration within the protein-protein complex will be investigated by controlled electrochemical addition of reducing equivalents and spectral monitoring of the redox states of the iron-sulfur and flavin chromophores.